The broad goal of this proposal is to understand the roles of Bone Morphogenetic Protein (BMP) signaling during inner ear development. This signaling pathway is crucial to the ontogeny of the bony and membranous labyrinths. However, the specific roles that BMP signaling plays in the chondrogenesis of the temporal bone, and in the formation of the semicircular canals and vestibular sensory epithelia are poorly understood. Thus, to better characterize the role of this pathway during inner ear ontogeny, we have developed several molecular genetic tools for analysis of BMP signaling during inner ear development. We will use Cre-mediated inactivation of type I BMP receptor genes to study BMP signaling during inner ear embryo genesis; employ transcriptional regulatory elements of the Brn4 gene to direct the expression of genes to the otic mesenchyme of transgenic mice; and exploit a pedigree of mice that mediates Cre-induced gene rearrangements in the otic mesenchyme to address the following specific aims: Aim #1. To characterize phospho-SMAD expression during inner ear development, we propose to undertake pan immunohistochemical analysis of phospho-SMAD localization to identify regions of active BMP signaling during inner ear development. Aim #2. To genetically test the role of BMP signaling during temporal bone formation, we will inactivate thepfunction of both BMP type I receptor genes, Bmprla and Bmprlb, in the otic mesenchyme. Functional analyses of these mutants will be assessed by recording auditory brain responses, endocochlear potential and middle ear transfer function with laser interferometer. Aim #3. To genetically test the role of BMP type I receptor genes during the ontogeny of sensory and nonsensorypepithelia, we will knockout these receptors in the otic vesicle. Analysis of mutant phenotypesporomises to elucidate fundamental molecular mechanisms governing inner ear development.